This invention relates to the manufacture of soft, absorbent tissue paper webs and particularly to the mode of creping of such webs to attain adequate softness and adhesive characteristics in the web while minimizing operational difficulties.
It is known in the art to form a thin paper web from a slurry of water and fiber, to dewater and at least partially dry the web and then present the same on a carrying fabric to a large steam-heated rotary drum termed in the art a Yankee dryer. The web commonly enters the dryer at a circumferential dryer position which is well around the dryer from the zone of web exit and the exit zone is equipped with a creping blade against which the web abuts to be pushed backwardly upon itself to attain the well known tissue crepe structure.
The creping action requires that the web be well adhered to the dryer to effect a consistent and uniform creping action, and for example, to prevent flaring of the web from the dryer before or at the zone of the creping blade. In some instances the web is presented to the dryer at a considerable moisture content that is as high as about 60%. Such web has a fiber consistency then upon dryer entry of about 40%. This moisture content depending upon the condition of the web surface and the Yankee dryer surface may tend to cause the web to adhere strongly to the dryer throughout the drying action. Under such circumstances, there is usually no requirement for the use of a supplemental adhesive such as the acetates or alcohols and in fact, on some occasions the adhesion to the dryer may be so tight that a release agent rather than an adhesive is applied between the dryer and the web to limit the extent of the adhesion.
In some modes of operation commonly referred to as through-drying the contact of the drying web with the dryer surface is limited. In a through-drying operation, the web formed from the slurry of water and fiber is dewatered without significant pressing of the wet web and this is followed by a drying action in a hot air blast. Such webs are then pressed to the Yankee dryer using a knuckled fabric so that the web adheres to the dryer in closely spaced zones, with bulking of the web between the zones. Fabrics having as fine a count as 4,900 openings per square inch and above may serve the purpose. The fiber consistency of such webs when presented to the dryer may be between about 30 and 90%. The higher fiber consistency webs will commonly require, and others may require, an adhesive to adequately secure the web to the dryer for both the completion of the drying action and for the creping action.
A variety of adhesives have been employed for retaining a web on a dryer surface. These include the polyvinyl acetate-ethylene copolymer emulsions and aqueous polyvinyl alcohol solutions. It has been found that the polyvinyl acetate-ethylene copolymer compositions (which may contain small percentages of polyvinyl alcohol, that is less than about 5% of the total solids by weight) are generally adequate for the purpose but have a number of undesirable effects.
Similarly, polyvinyl alcohol characterized by a degree of hydrolysis, has certain limitations tending to obviate it's use as the sole adhesive for retention of the web.
With more specific reference to the polyvinyl acetate-ethylene copolymers such tend to be retained on the creped paper web on the side which engages the dryer and when the creped web is rolled or rolled and plied with another web to form a sheet the surface or surfaces carrying the residual acetate tend to adhere causing blocking in the roll. Such blocking has additional detrimental effects in light weight tissues for if any minor edge defect or small hole occurs in a sheet the effort to separate the blocked plies results in tear outs from the web, complete web breaks and even ply reversal, that is the attachment of a ply to an adjacent sheet.
Additionally, it has been found that the fabric which carries the web to the dryer may become contaminated with the copolymer and this is difficult to remove with conventional cleaning actions due to the resistance to water of the copolymers.
In contrast to the polyvinyl acetate-ethylene copolymer the polyvinyl alcohols having a significant degree of hydrolysis tend to be water soluble and also tend to be unaffected by most organic solvents. The polyvinyl alcohols (which may contain some residual polyvinyl acetate) tend to coat the dryer with a hard and uneven film which builds up as drying and creping proceed resulting in uneven creping. Additionally, to remove the hard film, a cleaning blade is frequently used against the dryer surface causing dryer surface wear.